Light Emitting Diode (LED) is a semiconductor light-emitting device fabricated by adopting P-N junction electroluminescence principle. LEDs have advantages such as being—polluting, high brightness, low power consumption, long service life, low working voltage, easy miniaturization, etc. Since its successful development in 1990s, the GaN-based LED gets luminance improved and has wider applications with continuous research and development. Much studies are made on improving LED light-emitting efficiency, mainly concerning such technologies as pattern substrate technology, distributed current blocking layer (i.e., current blocking layer), distributed Bragg reflector (DBR) structure, transparent substrate, surface roughening, photonic crystal technology, etc.
Referring to FIG. 1, a normal LED structure, comprising a substrate 100, a bottom-up-laminated N-type layer 101, a light-emitting area 102, a P-type layer 103, a current spreading layer 104, a P electrode 106, an N electrode 107 on the exposed surface of the N-type layer 101 and a back-plating reflecting layer 108 under the substrate 100. The light emitted from the light-emitting layer (as shown in 1a) can be extracted from the front side of the chip. Light 1b, however, due to total reflection, cannot be extracted from the front of the chip but from the side; Light 1c is directly extracted from the side; Lights 1d and 1e are extracted from the front side of the chip after reflection of the back-plating reflecting layer 108.
Referring to FIG. 2, an improved conventional LED structure, comprising a substrate 100, a bottom-up-laminated N-type layer 101, a light-emitting area 102, a P-type layer 103, a current spreading layer 104, a metal reflecting layer 105, a P electrode 106, an N electrode 107 on the exposed surface of the N-type layer 101 and a back-plating reflecting layer 108 under the substrate 100. The light emitted from the light-emitting layer (as shown in 1a) can be extracted from the front side of the chip. Light 1b, however, due to total reflection, cannot be extracted from the front of the chip but from the side; Light 1c is directly extracted from the side; Lights 1d and 1e are extracted from the front side of the chip after reflection of the back-plating reflecting layer 108; light 1f, due to the dual reflection of the metal reflecting layer 105 (normally, Al or Ag) and the back-plating reflecting layer 108 (normally, Al, Ag or DBR), is finally extracted from the front side of the chip. In the above two normal LED structures, most light from the light-emitting layer is extracted from the front of the chip and less from the side of the chip, leading to uneven light distribution of LED, over strong light emitting at axial direction at front side, uneven heat dissipation and small light emitting angle.